Extractors with continuous action are known, which comprise several vertical pipes, connected parallely and arranged in a circle, enclosed by a common casing for heating with steam. The solid material is distributed by gravity in the different pipes and moves downwardly in them in the form of a dense layer. The solvent is fed into the bottom portion of the pipes and flows upwardly out from their upper portion through individual manually controlled valves.
The drawbacks of this type of extractor are the low efficiency of the mass-exchange process because of the lack of mixing of the liquid and solid phases and the difficulty maintaining an equal discharge of solvent through each pipe.
Sectional extractor is known, in which several working sections are disposed one above the other, forming a vertical column, ending on the top with a head. Each section represents a casing-pipe element having a bottom in which there are arranged nozzles for introducing gas upwardly into an array of vertical pipes, which provides a pneumatic mixing of the phases. The phases without gas bubbles move downwards along the outside of the pipes, under which there are no nozzles. The space between the pipes in each section is used for separating the phases. This design is suitable for liquid-liquid extraction, but it cannot be used for solid substances.